Re: [Xen-devel] [PATCH v2] x86/paravirt: Don't make vcpu_is_preempted() a callee-save function

[Waiman Long <longman@xxxxxxxxxx>]

On 02/10/2017 11:35 AM, Waiman Long wrote:
> On 02/10/2017 11:19 AM, Peter Zijlstra wrote:
>> On Fri, Feb 10, 2017 at 10:43:09AM -0500, Waiman Long wrote:
>>> It was found when running fio sequential write test with a XFS ramdisk
>>> on a VM running on a 2-socket x86-64 system, the %CPU times as reported
>>> by perf were as follows:
>>>
>>>  69.75%  0.59%  fio  [k] down_write
>>>  69.15%  0.01%  fio  [k] call_rwsem_down_write_failed
>>>  67.12%  1.12%  fio  [k] rwsem_down_write_failed
>>>  63.48% 52.77%  fio  [k] osq_lock
>>>   9.46%  7.88%  fio  [k] __raw_callee_save___kvm_vcpu_is_preempt
>>>   3.93%  3.93%  fio  [k] __kvm_vcpu_is_preempted
>>>
>> Thinking about this again, wouldn't something like the below also work?
>>
>>
>> diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
>> index 099fcba4981d..6aa33702c15c 100644
>> --- a/arch/x86/kernel/kvm.c
>> +++ b/arch/x86/kernel/kvm.c
>> @@ -589,6 +589,7 @@ static void kvm_wait(u8 *ptr, u8 val)
>>      local_irq_restore(flags);
>>  }
>>  
>> +#ifdef CONFIG_X86_32
>>  __visible bool __kvm_vcpu_is_preempted(int cpu)
>>  {
>>      struct kvm_steal_time *src = &per_cpu(steal_time, cpu);
>> @@ -597,6 +598,31 @@ __visible bool __kvm_vcpu_is_preempted(int cpu)
>>  }
>>  PV_CALLEE_SAVE_REGS_THUNK(__kvm_vcpu_is_preempted);
>>  
>> +#else
>> +
>> +extern bool __raw_callee_save___kvm_vcpu_is_preempted(int);
>> +
>> +asm(
>> +".pushsection .text;"
>> +".global __raw_callee_save___kvm_vcpu_is_preempted;"
>> +".type __raw_callee_save___kvm_vcpu_is_preempted, @function;"
>> +"__raw_callee_save___kvm_vcpu_is_preempted:"
>> +FRAME_BEGIN
>> +"push %rdi;"
>> +"push %rdx;"
>> +"movslq  %edi, %rdi;"
>> +"movq    $steal_time+16, %rax;"
>> +"movq    __per_cpu_offset(,%rdi,8), %rdx;"
>> +"cmpb    $0, (%rdx,%rax);"
>> +"setne   %al;"
>> +"pop %rdx;"
>> +"pop %rdi;"
>> +FRAME_END
>> +"ret;"
>> +".popsection");
>> +
>> +#endif
>> +
>>  /*
>>   * Setup pv_lock_ops to exploit KVM_FEATURE_PV_UNHALT if present.
>>   */
> That should work for now. I have done something similar for
> __pv_queued_spin_unlock. However, this has the problem of creating a
> dependency on the exact layout of the steal_time structure. Maybe the
> constant 16 can be passed in as a parameter offsetof(struct
> kvm_steal_time, preempted) to the asm call.
>
> Cheers,
> Longman

One more thing, that will improve KVM performance, but it won't help Xen.

I looked into the assembly code for rwsem_spin_on_owner, It need to save
and restore 2 additional registers with my patch. Doing it your way,
will transfer the save and restore overhead to the assembly code.
However, __kvm_vcpu_is_preempted() is called multiple times per
invocation of rwsem_spin_on_owner. That function is simple enough that
making __kvm_vcpu_is_preempted() callee-save won't produce much compiler
optimization opportunity. The outer function rwsem_down_write_failed()
does appear to be a bit bigger (from 866 bytes to 884 bytes) though.

Cheers,
Longman



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